1. Field of the Invention
This invention relates generally to semiconductor processing systems, and more particularly to radio frequency (RF) power generation and distribution.
2. Description of the Background Art
Semiconductor processing systems are used in the fabrication of semiconductor devices. In a semiconductor processing system that employs a plasma, such as a plasma enhanced chemical vapor deposition (PECVD) system, radio frequency (RF) power is supplied to the system""s reactor to maintain a plasma therein. A semiconductor substrate in the reactor is exposed to the plasma and process gases, thereby performing a fabrication process on the substrate. In a PECVD system, for example, the fabrication process may be deposition of a thin film of dielectric material on the substrate.
As the integration and complexity of semiconductor devices increase, so does the sensitivity of fabrication processes to the supplied RF power. This results in undesired process changes, commonly referred to as process shifts, when the RF generator supplying the RF power is changed or when the level of the RF power is unstable. In some cases, a process shift occurs even when the RF generator is replaced with another of the same make and model. Because process shifts result in unreliable or defective devices, a technique for stabilizing the RF power generated by the RF generator and seen by the plasma is desirable.
The present invention relates to a method and apparatus for stabilizing the radio frequency (RF) power used in semiconductor device processing. The invention is suitable for use in a variety of semiconductor processing systems including those used for chemical vapor deposition, physical vapor deposition, and etching.
In one embodiment, an absorptive filter network is provided between an RF generator and a semiconductor processing reactor. The absorptive filter network includes an absorptive filter circuit which allows energies at a fundamental frequency to pass while absorbing energies at frequencies away from the fundamental frequency. An absorptive filter circuit is located on a node facing the reactor to isolate the RF generator from the effects of the non-linear loading presented by a plasma in the reactor. Another absorptive filter circuit is located on a node facing the RF generator to present a stable voltage waveform to the plasma. The absorptive filter network minimizes variations in the supplied RF power, and helps prevent fabrication processes from shifting.
In one embodiment, an absorptive filter circuit includes a resistive element and a parallel resonant circuit that resonates at the fundamental frequency. At the fundamental frequency, the parallel resonant circuit appears as a high impedance path to ground and does not affect the RF power. At frequencies away from the fundamental frequency, the parallel resonant circuit shunts energies to the resistive element, which dissipates the energies. By dissipating unwanted energies instead of merely rejecting them, the chance of RF power variations causing a process shift is greatly diminished.
In another embodiment, the absorptive filter circuit also includes a parallel-series resonant circuit for rejecting residual energies at frequencies that are away from the fundamental frequency.
The above described embodiments are suitable for use in applications where the supplied RF power has a single, fixed frequency. For applications requiring variable frequency RF power, a filter network having a dissipating circuit is used to minimize RF power variations. The dissipating circuit includes a high-pass filter and a resistive element. The high-pass filter is set at a cut-off frequency above the frequencies of interest. Energies at frequencies below the cut-off frequency (i.e., the frequency range of the supplied RF power) are allowed to pass, while energies at frequencies above the cut-off frequency are shunted to the resistive element, where the energies are dissipated. A low-pass filter in series with the RF generator and the reactor is also provided to reject residual energies at frequencies away from the cut-off frequency. The aforementioned filter network with a dissipating circuit may also be used in fixed frequency applications.
These and other features, embodiments, and advantages of the present invention will be readily apparent to persons of ordinary skill in the art upon reading the entirety of this disclosure, which includes the accompanying drawings and claims.